The present invention relates to a torque-limiting bolt-and-nut assembly which is capable of being tightened only to a selected torque, especially for use in securing pipe ends together.
Bolt-and-nut assemblies are used to secure pipe ends together at a pipe junction. In that regard, attention is directed to FIG. 1 depicting a conventional pipe junction holder 10 for securing together the ends of two pipes 12, 14. Such a holder, disclosed in U.S. Pat. No. 4,092,036, includes a plurality of bolts 16 (only one depicted) which connect a ring 18 to a flange 20 of one of the pipes 12. The ring 18 carries a plurality of inclined bolts 22 (only one shown). Each bolt is connected to a respective pipe pressing member 24 to force edges 26, 28 thereof against an outer periphery of the pipe 14. Each inclined bolt 22 includes a flange 30 which bears against a slant surface 32 of the ring 18. By manually rotating the drive heads 33 of the bolts 22, the pressing members 24 are tightened against the ring, causing the ring 18 to press a packing member 34 against the ends of the two pipes to sealingly secure the pipe ends together. The slant surface 32 is inclined relative to the pipe axis, such that if the pipe ends try to separate axially from one another, the pipe pressing members 24 will be urged more forcefully against the pipe 14 to more vigorously resist the separation.
It will be appreciated that if the inclined bolts 22 are excessively rotated by a technician, the pressing member 24 may be forced too hard against the pipe 14, producing damage thereto. To prevent that from occurring, it has been proposed to utilize bolts that possess a torque-limiting structure. Exemplary of such bolts are those disclosed in U.S. Pat. Nos. 3,595,124; 3,812,757; and 4,627,774, wherein the driving head of the bolt is designed to shear-off when a predetermined torque level is exceeded.
In addition, as shown in U.S. Pat. No. 4,627,744, and in FIGS. 2 and 3 herein, it is conventional to provide an overlying protective nut on the shearable drive head to prevent that shearable drive head from breaking off prematurely if subjected to an accidental impact. In that regard, and with reference to FIGS. 2 and 3, a bolt 40 includes a threaded shank 42, a collar 44, a cylindrical intermediate segment 46 extending through a respective hole 45 formed in the flange 30, a secondary head 48, a shearable drive head 50, and a retaining rivet 52 for retaining the protective nut 54 in place. The nut 54 includes a recess 56 configured correspondingly to the drive head 50 to transmit torque to the drive head when the nut is rotated by a turning tool (e.g., a wrench). The nut extends axially sufficiently far to enable a free end thereof to overlie a portion of the cylindrical intermediate segment 46.
Among the shortcomings of such prior art is the fact that if the rivet 52 becomes broken and the nut 54 becomes accidentally dislodged from the bolt 40, the turning torque necessary to rotate the drive head must be applied directly to the drive head. However, the drive head 50 has a cross-sectional shape which is not of the traditional shape for which standard wrenches are designed, (i.e., a rectangular shape formed by two long sides 57 joined by two short sides 58). Thus, the drive head is not adapted to be gripped by wrenches of the type normally found in the tool box of a typical technician. Since the technician will be located in the field where the pipes are to be joined, it may be difficult for the technician to locate a wrench that is adapted to the shape of the drive head. As a result, the technician may resort to applying torque to the secondary drive head 48 which has a more traditional shape, but which is not of the shearable type, thereby giving rise to the risk of overtightening the pipe pressing member, as discussed earlier herein in connection with FIG. 1.
Therefore, it would be desirable to provide a torque-limiting bolt-and-nut assembly having a shearable drive head which can be driven by tools that are traditionally found in technicians toolboxes. It would also be desirable to provide a simpler way of attaching a protective nut to the bolt of such an assembly.
Accordingly, the present invention provides a torque-limiting bolt-and-nut assembly. The bolt includes a connector portion including a threaded shank, a secondary drive head integrally joined to an end of the connector portion, and a shearable drive head integrally joined to the secondary drive head by a shear-off section designed to be sheared-off by a torque exceeding a predetermined value. A recess is formed in the bolt between the shearable drive head and the secondary drive head. A nut is affixed on the shearable drive head and is configured for transmitting torque to the shearable drive head from a tool.
Preferably, the nut includes a connection structure extending into the recess for resisting dislodgement of the nut from the bolt.
Also, preferably, the shearable drive head has at least four sides as viewed in a cross-sectional plane oriented perpendicular to a center axis of the bolt. All of the sides of the drive head are of equal length, and the number of sides is an even number.